Editorial: Algal symbiotic relationships in freshwater and marine environments

COPYRIGHT © 2023 Tirichine and Piganeau. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. TYPE Editorial PUBLISHED 20 February 2023 DOI 10.3389/fpls.2023.1155759

ascomycete fungi that provide nitrogen to the algae (Simon et al., 2017). Recently, similar artificial symbiosis was reported in the marine microalgae Nannochloropsis oceanica and a terrestrial fungus, Mortierella elongate. Here, an unusual interaction takes place as functional algal cells are included within fungal mycelium, while in all known algae-fungus interactions, the algal cells remained external to fungal hyphae (Du et al., 2018;Du et al., 2019). This study showed the stability of the interaction with a bidirectional exchange of nutrients suggesting this could be the beginning of an endosymbiogenesis within eukaryotes. Despite this recent recognition and striking importance, symbiosis received less attention compared to other fields of investigation and remains largely unexplored in particular in aquatic biota where the nature of the environment, fluid, represents an additional challenge. The advent of symbiosis as a critical area of research is challenged by the difficulties in maintaining symbiotic partners/holobiont alive in lab cultures, in particular for obligate interactions when both partners are dependent on each other for survival. Often, commonly used culturing techniques are not suitable to species in symbiotic relationships. The lack of knowledge about their genome background and metabolic capabilities, to predict their nutrient requirements and culture conditions, hinders the progress in this field. However, recent and rapid advances in whole genome sequencing of either individual species and/or meta-communities helped to overcome some of these bottlenecks, using data mining for designing custom based media that fulfill the needs of symbiotic organisms (Leon et al., 2014;Jaswal et al., 2019;Lugli et al., 2019). Another fundamental boost is undoubtedly the existence of established model organisms that open up novel avenues of investigations, which would otherwise be impossible or at least difficult to achieve. A recent study that used both metagenomics sequencing and a model species, the diatom Phaeodactylum tricornutum, successfully identified an overlooked symbiosis between microalgae and non-cyanobacteria diazotrophs (NCDs) which challenges the long-held paradigm of dominance of cyanobacteria interactions with microalgae over NCDs and brings the first hints on how heterotrophic proteobacteria thrive in surface waters and oxygenated areas (Chandola et al., 2022).
The articles included in this Research Topic provide a view of how symbiotic interactions can help bypass environmental stresses such as in Heo et al., where filamentous ascomycetes Arthrinium species act as endosymbionts protecting brown algae from oxidative stress. Miao et al. studied the diversity and function of gammarid like animals that feed on the blooming green tides of Ulva prolifera contributing to their containment and limiting their negative impact on the environment. Lo et al. address the methodological challenge of inferring phylogenetic relationships from large genome assemblies in the (1-5Gb) genomes of Dinoflagellates Symbodinium, essential symbionts of corals. Their in silico analyses demonstrate that a scalable k-mer approach largely agrees with the phylogenetic signal inferred from the LSU rDNA sequence. The combination of genomic and experimental data sometimes allows us to hypothesize about the metabolic bases of coexistence, as in the case of coexistence between Roseovarius and the green alga Ostreococcus tauri, where the bacterial genomes encodes the metabolic pathway to produce the vitamins needed by the microalgae (Vacant et al). This study reports a stable coexistence maintaining the microalgae and the bacterium over several years, unlike the dynamic associations reported between Dinoroseobacter shibae and the microalgae Prorocebtrum minimum (Mansky et al., 2021), or Sulfitobacter andEmiliania huxleyi (Barak-Gavish et al., 2023).
The field of symbiosis in aquatic habitats is expanding quickly with both in silico and experimental approaches, and many novel insights into species interactions, their ecology and evolution are expected in the near future.

Author contributions
All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.

Funding
LT acknowledges support from the region of Pays de la Loire (ConnecTalent EPIALG project).

Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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